So I have 4 cats both at 100% love partnered to each other and still no kitten. What is going on? One of the couples is really really old and never gave me a kitten the others 2 are just about 3 weeks old. Is this a bug or am I doing something wrong. All answers appreciated. Thank you!

100% love only is not enough for your cats to give a box.
They also have to show 2 hearts, which means their stats are as following :
100% love
at least 75% happiness
at least 25% energy and awake
minimum 25% hunger.

Once they both show 2 hearts, usually turning walk/run off on both cats of the pair and sitting them next to each other is enough.
If it is not (still no box after 10minutes), then resetting the cats' home point and making them jump home speeds up the process.